yeah, the english is very hard to me :p
You need add the global $topicinfo.....
$context['topic_first_message']) is for the first message in every page... but $topicinfo['id_first_msg'] is for the first post.
Fine, but in my case it didn't happen that way, it only shows in the opening post of a topic, instead of the first post of a new page. It works for me at least

, however, it's good to know another way, so I can learn much more

.